The present invention relates to a sensor for measuring an air-fuel ratio and, more particularly, to a gas sensor for measuring an air-fuel ratio which is used to control a flow rate of a fuel of an internal combustion engine and is suitable to measure an air-fuel ratio of an air-fuel mixture of the internal combustion engine. In general, in a fuel control system for automobiles using an air-fuel ratio measuring gas sensor, by measuring a concentration of oxygen (O.sub.2) or unburnt gas (H.sub.2, CO) in the exhaust gas, information regarding the air-fuel ratio is obtained, the information is fed back to an apparatus to control a supply amount of a fuel, that is, a gasoline and an air amount, and a mixture ratio of the air and gasoline, i.e., an air-fuel ratio (A/F) is controlled.
An example of air-fuel ratio measuring gas sensors has been disclosed in co-pending patent application Ser. No. 250,238, now U.S. Pat. No. 4,915,814, which was filed to the U.S.A. on Sept. 28, 1988, by the inventors some of whom are the same as the inventors of the present patent application. The conventional gas sensor mentioned above has a wide air-fuel ratio measurable range from a rich region to a lean region of the air-fuel ratio.
For such a gas sensor to detect the wide air-fuel ratio range, in order to obtain the accurate air-fuel ratio information, the sensor must be manufactured by accurately managing the area (catalyst active region) of a reactive electrode of the sensor. The reasons will now be described with reference to the drawings.
FIG. 1 is an external view showing an electrode position of a general gas sensor for measuring an air-fuel ratio. FIG. 2 is a vertical sectional view taken along the line II--II in FIG. 1 showing the air-fuel ratio measuring gas sensor of the limiting current type. FIG. 3 is a diagram showing electric characteristics in the cases where a glass insulative layer exists and where it does not exist.
In general, inside and outside reactive electrodes on a solid state electrolyte 1 of the gas sensor for measuring the air-fuel ratio are formed by plating platinum. As shown in FIG. 1, an outside reactive electrode 2b is formed like a ring. A stripe-shaped lead electrode 4 is connected to the reactive electrode 2. The reactive electrode 2b and lead electrode 4 are simultaneously formed by the platinum plating by masking the portions other than the electrode portions.
As shown in FIG. 2, an inside reactive electrode 2a is formed by plating platinum onto the whole surface of the inside of the solid state electrolyte 1. This is because it is very difficult to form the ring-shaped reactive electrode on the inside of the electrolyte 1 in a manner similar to the outside ring-shaped reactive electrode and to form the stripe-shaped lead electrode since the inner diameter of the inside of the electrolyte 1 is small. Therefore, the inside reactive electrode 2a is formed so as to also function as the inside lead electrode by plating platinum to the whole surface of the inside.
The principle of the air-fuel ratio measuring gas sensor will now be described with reference to FIGS. 2 to 4.
The oxygen gas existing on the exhaust gas side passes through a gas diffusion layer 3 and is ionized by the outside reactive electrode 2b due to the catalyst reaction. The oxygen ions O.sup.2- pass through the solid state electrolyte 1 and move to the atmosphere side. At this time, an amount of oxygen gas which passes is restricted by the gas diffusion film 3 and exhibits a saturation characteristic as shown by (a) in FIG. 3.
FIG. 4 shows an enlarged diagram of the portion which contributes to the reaction of the sensor. When the air-fuel mixture is set to a value on the lean side, oxygen is dominant among the components in the exhaust gas. The oxygen molecules pass through the gas diffusion layer 3 and reach the reactive electrode 2b. The reactive electrode 2b is made of porous platinum and has a number of holes as shown in the diagram. In the hole portion, a three-phase interface in which platinum, solid state electrolyte 1, and gas molecules simultaneously exist is formed. The catalyst function occurs at the three-phase interface. According to the reaction at the three-phase interface when the air-fuel mixture is lean, the oxygen molecules which pass through the diffusion layer 3 react to the electrons which are provided from the electrode and the following reaction occurs. EQU O.sub.2 +4e.sup.2- .fwdarw.2O.sup.2-
The oxygen ions generated pass through the solid state electrolyte 1 having a potential gradient and reach the inside reactive electrode 2a, so that a pump current is caused.
When the air-fuel mixture is rich, the main components in the exhaust gas are hydrogen, carbon monoxide, and hydrocarbon. In this case, the oxygen molecules on the atmosphere side are ionized by the inside reactive electrode 2a and pass through the electrolyte 1 and reach the three-phase interface of the outside reactive electrode 2b. On the other hand, the molecules of H.sub.2, CO, and HC in the exhaust gas pass through the gas diffusion layer 3 and reach the three-phase interface of the outside reactive electrode 2b and react to the oxygen ions as follows. EQU CO+O.sup.2- .fwdarw.CO.sub.2 +2e.sup.- EQU H.sub.2 +O.sup.2- .fwdarw.H.sub.2 O+2e.sup.-
2HC+5O.sup.2- .fwdarw.2CO.sub.2 +H.sub.2 O+10e.sup.-
Thus, a pump current is caused. The direction of the current in this case is opposite to that in the case where the air-fuel mixture is lean.
In the portion where a glass insulative layer 8 covers the electrode 2b, the molecules or the molecules of hydrogen, carbon monoxide, and hydrocarbon cannot pass through the in of the glass, so that the catalyst function by the three-phase interface is not effected.
FIG. 3 shows a limiting value by a solid line in which the pump current constant. In FIG. 3, an axis of abscissa denotes a voltage V between electrodes and an axis of ordinate indicates a pump current I.sub.p. When the air-fuel ratio A/F is changed, the limiting current value corresponding to the relevant air-fuel ratio is obtained.
The limiting current value is proportional to the area (effective area of the holes of the reactive electrode) of the reactive electrode. Therefore, to obtain the gas sensor having the output which accurately corresponds to the air-fuel ratio, the reactive electrodes must be formed by accurately managing the area of the outside reactive electrode 2b which faces the inside reactive electrode 2a through electrolyte 1.
However, the outside lead electrode 4 is formed by platinum having the same catalyst function as that of the outside reactive electrode 2b and also faces the inside reactive electrode 2a. Therefore, a pump current due to the oxygen ions also flows even in the portion where the lead electrode 4 faces the inside reactive electrode 2a. The value of the pump current in the lead electrode portion has a very large variation and gives a measurement error because the temperature distribution in the lead electrode portion by a heater in the sensor is not uniform. (The temperature is also one of the parameters which changes the pump current.)
Therefore, with respect to the outside lead electrode 4, the catalyst function thereof needs to be restricted so as not to contribute to the pump current, or the oxygen molecules or unburnt gas molecules (H.sub.2, CO, HC) need to be blocked so as not to reach the lead electrode surface. The glass insulative layer 8 shown in FIG. 2 has such a function as to block the gas molecules.
In the case where the glass insulative layer 8 shown in FIG. 2 does not exist, the current characteristic of the sensor does not have a limiting current value as shown by a broken line in (b) in FIG. 3. This is because, as mentioned above, when the glass insulative layer 8 does not exist, the gas molecules are also ionized even on the lead electrode 4 by the catalyst reaction (In the lean region, the oxygen molecules emit the electrons and become the oxygen ions and pass through the solid state electrolyte 3 and reach the inside electrode 2a. In the rich region, the molecules of hydrogen, carbon monoxide, and hydrocarbon are ionized and are combined with the oxygen ions which passed through the electrolyte 3.) and a pump current flows and is added to the pump current which is caused due to the outside reactive electrode 2b.
Therefore, it is extremely important to prevent the ionization of the gas molecules at the lead electrode 4 and to prevent the gas molecules which propagate on the surface of the lead electrode and enter the reactive electrode 2b.
In JP-A-58-24855 which has been filed to Japanese Patent Office on Aug. 5, 1981, by Nippondenso Co., Ltd. and has been laid open on Feb. 14, 1983, there has been disclosed a construction such that after a lead electrode was covered by a coating film having a glass impermeability of a glass of a high melting point, a gas diffusion layer is formed by the plasma flame coating.
In JP-A-61-45962 which has been filed in the Japanese Patent Office on Aug. 9, 1984, by Toyota Central Research & Development Labs., Inc. and has been laid open on Mar. 6, 1986, there has been disclosed a construction in which an insulative layer is formed between a lead electrode and a solid state electrolyte.
In JP-A-58-2662(U) which has been filed in the Japanese Patent Office on June 29, 1981, by Nihon Denshi Kiki Kabushiki Kaisha, there has been disclosed a construction in which outside and inside lead electrodes of a solid state electrolyte are formed at different positions.
Like a technique disclosed in JP-A-58-24855, a glass having a high melting point is formed as a glass insulative layer onto the lead electrode of platinum. In the manufacturing processes, after a glass of a high melting point was coated onto the platinum lead electrode, it is sintered at about 600.degree. C. by using an electric furnace or the like. Therefore, the adhesive property between platinum and glass of a high melting point is bad and their coefficients of thermal expansion largely differ. Thus, it is very difficult to form a thin film. On the contrary, it is considered that it is difficult to cover the whole surface of the lead electrode unless the film is formed thickly.
On the other hand, in the next step, to form the gas diffusion layer onto the glass insulative layer or the like, pulverulent magnesiaspinel is used as a material and the gas diffusion layer is formed by the plasma flame coating. In this case, magnesiaspinel having a melting point of about 2100.degree. C. is supplied into the plasma jet and is semi-fused and is struck against the glass insulative layer at a high speed, thereby forming the gas diffusion layer. Thus, there is a problem such that the glass of a high melting point which is used for the glass insulative layer causes a crack by the influence when magnesiaspinel was struck against the glass at a high speed. Therefore, there occur problems of the ionization of the oxygen gas on the lead electrode which occurs due to the crack existing in the glass insulative layer and an erroneous current which is caused by the oxygen gas which enters from the upper portion of the lead electrode. Thus, there is a problem such that air-fuel ratio cannot be accurately detected.
On the other hand, even the technique disclosed in JP-A-61-45962 relates to the construction such that the glass insulative layer is formed by the plasma flame coating and also has a problem similar to that in the technique disclosed in JP-A-58-24855. The technique disclosed in JP-U-58-2662 has a problem such that the working step to form the inside and outside electrodes at deviated positions is complicated and the producing costs are high.